                1. Field of Invention        
The field of the currently claimed embodiments of this invention relates to systems and components for stimulating nerves, and more particularly to systems that include surgically implantable vestibular prostheses and components, algorithms, stimulus protocols and methods for surgically implantable vestibular prostheses.                2. Discussion of Related Art        
In normal individuals, the two inner ear labyrinths modulate activity on afferent fibers within each vestibular nerve branch so as to provide the central nervous system (CNS) with sensation of rotational head motion and linear accelerations due to both gravity and translational motion (termed gravitoinertial acceleration). Each labyrinth contains three mutually orthogonal semicircular canals (SCCs) to sense head rotation. Each SCC modulates activity on its branch of the vestibular nerve approximately in time with the component of 3-dimensional (3D) head angular velocity about the axis of that SCC. (See FIG. 1.) Each SCC is approximately coplanar with an SCC in the opposite ear, and each coplanar pair of SCC effectively acts as a pair of antiparallel angular rate sensors. The SCCs, oriented in the horizontal, left-anterior-right-posterior (LARP), and right-anterior-left-posterior (RALP) axes, are responsible for sensing angular velocity in those respective axes, and the two otolith end organs (the utricle and saccule) are responsible for sensing gravitoinertial (translational) accelerations. These sensory inputs drive compensatory reflexes that stabilize gaze and posture so as to maximize clarity of vision during head movement and to prevent falls. Patients who have lost vestibular hair cell function in both labyrinths can suffer from debilitating loss of visual acuity and balance because their CNS no longer receives normal head movement information or gravitational orientation cues. While compensatory use of visual and proprioceptive input might partially supplant lost labyrinthine input, this strategy fails during high frequency, high acceleration, transient head motions, such as those experienced while walking (Carey, J. P. and C. C. Della Santina. Principles of applied vestibular physiology. Otolaryngology—Head & Neck Surgery. 2005). Approximately 0.1% of U.S. adults report a constellation of symptoms consistent with severe bilateral vestibular hypofunction, corresponding to more than 250,000 individuals in the U.S. alone (Della Santina, C. C., A. A. Migliaccio, R. Hayden, T. A. Melvin, G. Y. Fridman, B. Chiang, N. S. Davidovics, C. Dai, J. P. Carey, L. B. Minor, I. C. W. Anderson, H. Park, S. Lyford-Pike, and S. Tang. Current and future management of bilateral loss of vestibular sensation—an update on the Johns Hopkins multichannel vestibular prosthesis project. Cochlear Implants International. 2010). For those who fail to compensate through rehabilitation exercises, no adequately effective treatment exists. A multichannel vestibular prosthesis that directly modulates activity of surviving vestibular afferents based on motion sensor input could improve quality of life for vestibular-deficient individuals if it effectively restores sensation of head motion and gravitational orientation (Della Santina et al., supra; Wall, C., D. M. Merfeld, S. D. Rauch, and F. O. Black. Vestibular prostheses: The engineering and biomedical issues. Journal of Vestibular Research-Equilibrium & Orientation. 12: 2002).
Gong and Merfeld described the first head-mounted vestibular prosthesis in 2000 (Gong, W. S. and D. M. Merfeld. Prototype neural semicircular canal prosthesis using patterned electrical stimulation. Annals of Biomedical Engineering. 28: 2000; Gong, W. S. and D. M. Merfeld. System design and performance of a unilateral horizontal semicircular canal prosthesis. IEEE Transactions on Biomedical Engineering. 49: 2002; Merfeld et al U.S. Pat. No. 6,546,291 B2). That device is capable of sensing head rotation about one axis and electrically stimulating the vestibular nerve via a pair of electrodes intended to excite afferents in an ampullary nerve innervating one SCC. Using this device, Gong, Merfeld et al. were able to partially restore the Vestibulo-Ocular Response (VOR) about one axis in squirrel monkeys and guinea pigs. They have since described long-term changes in the prosthetically-evoked VOR, postural effects, and responses to simultanenous, bilateral stimulation of the lateral SCCs (Gong, W. S., C. Haburcakova, and D. M. Merfeld. Vestibulo-Ocular Responses Evoked Via Bilateral Electrical Stimulation of the Lateral Semicircular Canals. IEEE Transactions on Biomedical Engineering. 55: 2008; Gong, W. S. and D. M. Merfeld. Prototype neural semicircular canal prosthesis using patterned electrical stimulation. Annals of Biomedical Engineering. 28: 2000; Gong, W. S. and D. M. Merfeld. System design and performance of a unilateral horizontal semicircular canal prosthesis. IEEE Transactions on Biomedical Engineering. 49: 2002; Lewis, R. F., W. S. Gong, M. Ramsey, L. Minor, R. Boyle, and D. M. Merfeld. Vestibular adaptation studied with a prosthetic semicircular canal. Journal of Vestibular Research-Equilibrium & Orientation. 12: 2002; Lewis, R. F., D. M. Merfeld, and W. S. Gong. Cross-axis vestibular adaptation produced by patterned electrical stimulation. Neurology. 56: 2001; Merfeld, D. M., W. S. Gong, J. Morrissey, M. Saginaw, C. Haburcakova, and R. F. Lewis. Acclimation to chronic constant-rate peripheral stimulation provided by a vestibular prosthesis. IEEE Transactions on Biomedical Engineering. 53: 2006; Merfeld, D. M., C. Haburcakova, W. Gong, and R. F. Lewis. Chronic vestibulo-ocular reflexes evoked by a vestibular prosthesis. IEEE Transactions on Biomedical Engineering. 54: 2007).
Della Santina et al. (Della Santina, C. C., A. A. Migliaccio, and A. H. Patel. Electrical stimulation to restore vestibular function—development of a 3-D vestibular prosthesis. 27th Annual IEEE Engineering in Medicine and Biology. 2005; Della Santina, C. C., A. A. Migliaccio, and A. H. Patel. A multichannel semicircular canal neural prosthesis using electrical stimulation to restore 3-D vestibular sensation. IEEE Transactions on Biomedical Engineering. 54: 2007) described a multichannel vestibular prosthesis (here denoted MVP1, for Multichannel Vestibular Prosthesis, version 1) capable of sensing angular velocity about three orthogonal axes and asynchronously stimulating each of the three ampullary nerves of a single labyrinth, allowing partial restoration of VOR responses for head rotation about any axis. Increasing the number of stimulating electrodes and the current amplitude resulted in spatial current spread within the implanted labyrinths which limited the ability to selectively stimulate the appropriate bundle of vestibular afferents. Increasing current amplitude initially increased the VOR magnitude without changing the intended rotational axis, but at higher amplitudes, the eye rotation axis deviated from ideal for the target SCC, as current spread to other bundles of vestibular afferents. Subsequent studies by Della Santina et al. have used the MVP1 to study optimization of stimulus coding strategy, a coordinate system orthogonalization approach to minimizing 3D misalignment errors, effects of vestibular electrode implantation on hearing, and changes in 3D VOR alignment during chronic prosthetic stimulation (Della Santina, C. C., A. A. Migliaccio, R. Hayden, T. A. Melvin, G. Y. Fridman, B. Chiang, N. S. Davidovics, C. Dai, J. P. Carey, L. B. Minor, I. C. W. Anderson, H. Park, S. Lyford-Pike, and S. Tang. Current and future management of bilateral loss of vestibular sensation—an update on the Johns Hopkins multichannel vestibular prosthesis project. Cochlear Implants International. 2010; Chiang, B., G. Y. Fridman, and C. C. Della Santina. Enhancements to the Johns Hopkins Multi-Channel Vestibular Prosthesis Yield Reduced Size, Extended Battery Life, Current Steering and Wireless Control. Association for Research in Otolaryngology. 2009; Davidovics, N., G. Y. Fridman, and C. C. Della Santina. Linearity of Stimulus-Response Mapping During Semicircular Canal Stimulation using a Vestibular Prosthesis. ARO 2009. 2009; Della Santina, C. C., A. A. Migliaccio, and L. B. Minor. Vestibulo—ocular reflex of chinchilla during high frequency head rotation and electrical stimuli. Society for Neuroscience Abstract Viewer and Itinerary Planner. 2003: 2003; Della Santina, C. C., A. A. Migliaccio, H. J. Park, I. C. W. Anderson, P. Jiradejvong, L. B. Minor, and J. P. Carey. 3D Vestibuloocular reflex, afferent responses and crista histology in chinchillas after unilateral intratympanic gentamicin. Association for Research in Otolaryngology Annual Mtg. 2005; Della Santina, C. C., A. A. Migliaccio, and A. H. Patel. Electrical stimulation to restore vestibular function—development of a 3-D vestibular prosthesis. 27th Annual IEEE Engineering in Medicine and Biology. 2005; Della Santina, C. C., A. A. Migliaccio, and A. H. Patel. A multichannel semicircular canal neural prosthesis using electrical stimulation to restore 3-D vestibular sensation. Ieee Transactions on Biomedical Engineering. 54: 2007; Della Santina, C. C., V. Potyagaylo, A. A. Migliaccio, L. B. Minor, and J. P. Carey. Orientation of human semicircular canals measured by three-dimensional multiplanar CT reconstruction. Jaro-Journal of the Association for Research in Otolaryngology. 6: 2005; Fridman, G. Y., N. Davidovics, C. Dai, and C. C. Della Santina. Multichannel Vestibular Prosthesis Stabilizes Eyes For Head Rotation About Any Axis. Journal of the Association for Research in Otolaryngology. Submitted 2009: 2009; Tang, S., T. A. N. Melvin, and C. C. Della Santina. Effects of semicircular canal electrode implantation on hearing in chinchillas. Acta Oto-Laryngologica. 129: 2009). Della Santina and Faltys described a hybrid cochlear and vestibular stimulator.
Shkel et al, Constandinou et al, and Phillips et al have also described vestibular prosthesis circuits but have not published results obtained from physiological testing (Shkel, A. M. and F. G. Zeng. An electronic prosthesis mimicking the dynamic vestibular function. Audiology and Neuro-Otology. 11: 2006; Constandinou, T. and J. Georgiou. A micropower tilt processing circuit. Biomedical Circuits and Systems Conference, 2008.BioCAS 2008.IEEE. 2008; Constandinou, T., J. Georgiou, and C. Andreou. An ultra-low-power micro-optoelectromechanical tilt sensor. Circuits and Systems, 2008.ISCAS 2008.IEEE International Symposium on. 2008; Constandinou, T., J. Georgiou, C. Doumanidis, and C. Toumazou. Towards an Implantable Vestibular Prosthesis: The Surgical Challenges. Neural Engineering, 2007.CNE '07.3rd International IEEE/EMBS Conference on. 2007; Constandinou, T., J. Georgiou, and C. Toumazou. A fully-integrated semicircular canal processor for an implantable vestibular prosthesis. Electronics, Circuits and Systems, 2008.ICECS 2008.15th IEEE International Conference on. 2008; Constandinou, T., J. Georgiou, and C. Toumazou. A Neural Implant ASIC for the Restoration of Balance in Individuals with Vestibular Dysfunction. IEEE International Symposium on Circuits and Systems (ISCAS). 2009; Constandinou, T., J. Georgiou, and C. Toumazou. A Partial-Current-Steering Biphasic Stimulation Driver for Vestibular Prostheses. Biomedical Circuits and Systems, IEEE Transactions on. 2: 2008; Phillips, J., S. Bierer, A. Fucks, C. Kaneko, L. Ling, K. Nie, T. Oxford, and J. Rubinstein. A multichannel vestibular prosthesis based on cochlear implant technology. Society for Neuroscience. 2008). Shkel et al described a custom-designed micro-electro-mechanical system (MEMs) gyroscope and a hardware-based solution for setting the pattern of electrical stimulation. Instead of using a microcontroller to determine pulse timing, Shkel et al developed a control circuit, which emulated the transfer function of SCC canal dynamics determined experimentally by Fernandez, Goldberg, et al (Baird, R. A., G. Desmadryl, C. Fernandez, and J. M. Goldberg. The Vestibular Nerve of the Chinchilla 0.2. Relation between Afferent Response Properties and Peripheral Innervation Patterns in the Semicircular Canals. Journal of Neurophysiology. 60: 1988). Constandinou et al described a vestibular prosthesis Application Specific Integrated Circuit (ASIC) and corresponding ASIC components, which could result in a smaller implant. As was the case with Shkel et al's device, the control circuit used in Constandinou et al's device is a circuit realization of the canal dynamics transfer function. To date, no physiological animal experiments have been reported by either Shkel et al or Constandinou et al. Phillips et al described a commercially available cochlear implant modified for use as a vestibular prosthesis.
All prosthetic vestibular nerve stimulation studies to date have encountered performance constraints due to suboptimal electrode-nerve coupling and selectivity, as well as limitations related to device size and power consumption. No vestibular prosthesis yet described and produced has included sensors of both rotation and gravitoinertial/translational acceleration or multiple current sources able to support multipolar “current steering” stimulus paradigms, nor has any yet achieved sufficient combination of miniaturization, system integration, multidimensional sensing, in situ self-testing ability and reduction in power consumption to constitute a prosthesis appropriate for long-term restoration of the VOR in vestibular-deficient patients.
The six semicircular canals located in the two inner ears (three in each ear) provide balance information to the brain by sensing the rotation of the head about three orthogonal axes, corresponding to the spatial orientation of each of the canals. A vestibular prosthesis can emulate this function by sensing the 3D rotation and linear acceleration of the head with three orthogonally oriented gyroscopes and linear accelerometers. The sensation of head rotation is transmitted to the brain by electrically stimulating the three corresponding branches of the vestibular nerve that normally carry such information from each of the semicircular canals in the implanted ear. The sensation of head linear acceleration is transmitted to the brain by electrically stimulating the three corresponding branches of the vestibular nerve that normally carry such information from the utricle and saccule in the implanted ear.
Recent advances in the development of vestibular prostheses demonstrated that current spread can severely degrade the precision with which the prosthesis can selectively target each of the branches of the vestibular nerve. Functionally, current spread causes misalignment between the sensed axis of head rotation and the axis of rotation that is conveyed via the electrical stimulation delivered to the vestibular nerve. This is because the stimulation current which is intended to deliver stimulation to only one of the branches of the nerve can spread to the neighboring branches, unintentionally stimulating them as well. The amount of current spread depends upon proximity of the electrode to the targeted nerve branch and path of the electrical current flowing through the tissue during stimulation. Thus accurate surgical placement of the electrode contact in close proximity to each of the intended stimulation sites and away from the untargeted branches of the nerve is critical to the operation of the prosthesis. Because the branches of the vestibular nerve are very near each other, such surgical placement can be difficult without causing damage to the delicate neural structures (ampullae), where the vestibular nerve enters the semicircular canals (SCCs). These entry points are targeted for electrical stimulation in each canal (FIG. 1).
There thus remains a need for improved implantable vestibular prostheses that facilitate accurate placement of electrodes and precise delivery of stimulus current while minimizing difficulty and variability of surgical implantation.